Calibrator/Control for Simultaneous Assay of Proteins Capable of Complexing With One Another

ABSTRACT

Disclosed herein are compositions and methods comprising two or more proteins in which at least one of the proteins has been altered to reduce their mutual recognition and binding. Such compositions are useful as reference, calibrators or controls in methods and assays for determining the amount of one or more of the proteins that may be present in a sample of interest or in confirming the presence of one or more of the proteins in the sample. More particularly, it relates to compositions and methods comprising altered placental growth factor-1 (PlGF-1) and soluble fms-like tyrosine kinase (sFlt-1) and methods for determining the amount or confirming the presence of sFlt-1 and/or PlGF-1 in a sample of interest.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/349,695, filed Jan. 7, 2009, which claims the benefit of U.S.Provisional Application No. 61/019,443, filed Jan. 7, 2008.

FIELD OF THE INVENTION

The present invention relates to compositions and methods comprising twoor more proteins altered to prevent their mutual recognition andbinding. The compositions can be used as reference, calibrator orcontrol in analytical assays capable of detecting both altered andunaltered or native forms of one or more of the proteins.

BACKGROUND OF THE INVENTION

Pre-eclampsia is a syndrome of hypertension, edema, and proteinuria thataffects 5 to 10% of pregnancies and results in substantial maternal andfetal morbidity and mortality. Pre-eclampsia accounts for at least200,000 maternal deaths worldwide per year. The symptoms ofpre-eclampsia typically appear after the 20^(th) week of pregnancy.

Development of a fetus and placenta is mediated by several growthfactors. Vascular endothelial growth factor (VEGF) is an endothelialcell-specific mitogen, and angiogenic inducer. VEGF mediates vascularpermeability and has been shown to be involved in glomerular capillaryrepair. VEGF binds as a homodimer to one of two homologousmembrane-spanning receptor tyrosine kinases, the fms-like tyrosinekinase (Flt-1) and the kinase domain receptor (KDR).

Placental growth factor (PlGF) is a VEGF family member that is alsoinvolved in placental development. PlGF is expressed by cytotrophoblastsand syncytiotrophoblasts and is capable of inducing proliferation,migration, and activation of endothelial cells. PlGF binds as ahomodimer to the Flt-1 receptor, but not the KDR receptor. Both PlGF andVEGF contribute to the mitogenic activity and angiogenesis that arecritical for the developing placenta.

A soluble form of the Flt-1 receptor (sFlt-1) has been identified in acultured medium of human umbilical vein endothelial cells and in vivoexpression was subsequently demonstrated in placental tissue. sFlt-1 isa splice variant of the Flt-1 receptor which lacks the transmembrane andcytoplasmic domains. sFlt-1 binds to VEGF with high affinity but doesnot stimulate mitogenesis of endothelial cells. sFlt-1 is believed toact as a “physiologic sink” to down-regulate VEGF signaling pathways.Regulation of sFlt-1 levels therefore works to modulate VEGF and VEGFsignaling pathways. Careful regulation of VEGF and PlGF signalingpathways is critical for maintaining appropriate proliferation,migration, and angiogenesis by trophoblast cells in the developingplacenta.

A single gene codes for human PlGF. However, splicing of the mature PlGFmRNA results in three different length isoforms: PlGF-1 (PlGF131),PlGF-2 (PlGF152), and PlGF-3 (PlGF203). Another variant, PlGF-4, hasbeen reported (Yang, et al, J Reprod Immunol, v 60, p 53-60, 2003). PlGFis secreted as a glycosylated homodimer.

Recently it has been shown that sFlt-1 and PlGF may be used individuallyor in combination as biomarkers to predict, diagnose, or monitorpre-eclampsia (Levine et al, NEJM, v 350, p 672-683, 2004).

The amino acid sequence of mature human PlGF-1, amino acid residues1-132, has been published and is available from the Protein Data Bankidentified as PDB 1FZV (Iyer, et al, J. Biol Chem, v 276, p 12153-12161,2001). This sequence is identified herein as SEQ ID NO:1:

MLPAVPPQQW ALSAGNGSSE VEVVPFQEVW GRSYCRALERLVDVVSEYPS EVEHMFSPSC VSLLRCTGCC GDENLHCVPVETANVTMQLL KIRSGDRPSY VELTFSQHVR CECRPLREKM KPERCGDAVP RR

Diagnosis of an individual at risk for, or having pre-eclampsia may bemade by determining the presence or amount of vascular endothelialgrowth factor, particularly PlGF, and/or receptor tyrosine kinase,particularly, sFlt-1 in a biological sample (such as urine, whole blood,serum, plasma, saliva, and so forth) taken from the individual. Inanalytical assays reference, calibrator and control compositions areessential for purposes of determining the amount or confirming thepresence of a target analyte and, for establishing accuracy andprecision of the analytical assay. The preparation of such compositionsin liquid or dry form usually doesn't present difficulties if theanalyte is readily available, soluble in an appropriate solvent—usuallyaqueous for biological analytes, stable, and does not interactdeleteriously with other components that may be present in thecomposition. As noted above, PlGF binds sFlt-1 to form a stableassociation complex. As a result compositions comprising the nativeproteins together in independent amounts suitable for use as areference, calibrator or control in analytical assays to detect PlGF orsFlt-1 or both PlGF and sFlt-1 cannot be prepared. Although compositionscomprising the individual, separated proteins may be prepared it wouldbe advantageous to be able to prepare compositions comprising bothproteins together. Thus, a need exists for reference, calibrator orcontrol compositions comprising these proteins together in known andindependent amounts. This need has been met with the present invention.

SUMMARY OF THE INVENTION

In one aspect the present invention relates to a composition comprisingtwo or more proteins, one or more of the proteins having been altered tosufficiently reduce or substantially prevent or eliminate mutualrecognition and binding. Such a composition is useful as a reference,calibrator or control in analytical assays for one or more of theproteins in the composition.

Considering for clarity two proteins unaltered/native proteins A and Bwhich form a non-covalent association complex, the term “substantiallyprevent or eliminate their mutual recognition and binding” means that inan assay to determine their mutual binding, binding of altered A tounaltered/native B, or binding of unaltered/native A to altered B, orbinding of altered A to altered B is not detectable, or barelydetectable, or the mutual affinity as measured quantitatively bydetermination of affinity constants is less than approximately 10% ofthat observed for unaltered/native A and unaltered/native B. The term“sufficiently reduce” means that mutual binding occurs, but it has beenreduced to a degree that is acceptable for a particular application.

Consider a case where the presence or amount of unaltered or nativeprotein A is to be determined in an analytical assay, which assayutilizes one or more receptors specific for epitopes of protein A. Andconsider that protein A has been altered to reduce or substantiallyeliminate binding to protein B. Although protein A has been altered theepitopes remain intact or acceptably intact such that they retain theirability to recognize and bind the receptors. Thereby, altered protein Ais acceptable for use in calibrating the assay, confirming the presenceof unaltered/native protein A in a sample, or for verifying the accuracyand precision of the assay for unaltered/native protein A. Thus, ingeneral, receptors are capable of recognizing and binding both alteredand unaltered/native forms of a protein. Analytical assays comprisingreceptors are usually immunoassays, which assays employ as receptorspolyclonal or monoclonal antibodies, whole, polymeric and/or chimericforms of antibodies or antibody fragments. Other kinds of receptors arealso used, such as aptamers (U.S. Pat. No. 5,840,867; U.S. Pat. No.6,207,388). In an analytical assay for the determination of unaltered ornative protein B using receptors specific for epitopes of protein B, itis not necessary that epitopes of altered protein A remain intact. It isonly important that mutual recognition and binding of altered protein Aand protein B have been sufficiently reduced or substantiallyeliminated.

If both protein A and protein B have been altered to reduce orsubstantially eliminate their mutual recognition and binding then in ananalytical assay for determination of unaltered/native protein A or ananalytical assay for determining unaltered/native protein B or ananalytical assay for determination of both unaltered/native proteins Aand B—which assays utilize receptors specific for epitopes of protein Aand receptors specific for epitopes of protein B, these epitopes in thealtered proteins retain the ability to recognize and bind the receptorsused in the assay.

Compositions comprising both altered protein A and altered protein Btogether can then be used for calibrating the assays, confirming thepresence of unaltered/native protein A, or unaltered/native protein B,or both unaltered/native protein A and unaltered/native protein B, andfor verifying accuracy and precision of the assays.

In another aspect the present invention relates to a reference,calibrator or control composition for use in an assay for a firstprotein or a second protein or both first and second proteins, whereinone or more amino acids or one or more non-amino acid groups of thefirst protein or the second protein or the first protein and the secondprotein have been deleted, modified, or replaced with a different aminoacid or non-amino acid group or groups thereby reducing or substantiallyeliminating mutual binding of the first protein and the second protein.

In another aspect the present invention relates to a compositioncomprising a receptor tyrosine kinase, preferably fms-like tyrosinekinase, more preferably sFlt-1 and a vascular endothelial growth factoreither or both altered, by amino acid or glycosyl deletion, modificationor replacement. The vascular endothelial growth factor may be aplacental growth factor and preferably, PlGF-1. A preferred compositioncomprises sFlt-1 and altered PlGF-1 having alanine in place of:

-   -   a) proline at position 25 of SEQ ID NO:1, or    -   b) glutamine at position 27 of SEQ ID NO:1, or    -   c) cysteine at position 60 of SEQ ID NO:1, or    -   d) aspartate at position 72 of SEQ ID NO:1 or    -   e) glutamate at position 73 of SEQ ID NO:1, or    -   f) asparagine at position 84 of SEQ ID NO:1, or    -   g) proline at position 98 of SEQ ID NO:1, or    -   h) tyrosine at position 100 of SEQ ID NO:1, or        altered PlGF-1 having glycine in place of cysteine at position        70 of SEQ ID NO:1, or any combination of the alanine        replacements in a) to h) and the glycine replacement.

A preferred composition comprises sFlt-1 and altered PlGF-1 havingalanine in place of aspartate at position 72 of SEQ ID NO:1 and alaninein place of glutamate at position 73 of SEQ ID NO:1.

Another preferred composition comprises sFlt-1 and altered PlGF-1 havingglycine in place of cysteine at position 70 of SEQ ID NO:1, alanine inplace of aspartate at position 72 of SEQ ID NO:1, and alanine in placeof glutamate at position 73 of SEQ ID NO:1.

In another aspect the present invention relates to a method forcalibrating an assay for a protein in a sample comprising the steps of:

-   -   1) contacting a composition as described above comprising known        amounts of the protein with a receptor specific for a first        epitope of the protein, thereby forming a complex comprising the        receptor and the protein of the composition;    -   2) contacting the complex formed in step 1) with a labeled        receptor specific for a second epitope of the protein, thereby        forming a complex comprising receptor, the protein of the        composition, and labeled receptor;    -   3) detecting a signal from bound labeled receptor or a signal        from free labeled receptor; and,    -   4) associating the signal from free or bound labeled receptor        with the known amounts of the protein in the composition.    -   In another aspect the invention relates to a method for        calibrating an assay for a protein in a sample comprising the        steps of:    -   1) contacting a composition as described above comprising known        amounts of the protein with an immobilized receptor specific for        a first epitope of the protein, thereby forming a complex        comprising immobilized receptor and the protein of the        composition;    -   2) contacting the complex formed in step 1) with a labeled        receptor specific for a second epitope of the protein, thereby        forming a complex comprising immobilized receptor, the protein        of the composition and labeled receptor;    -   3) separating bound labeled receptor from free labeled receptor;    -   4) detecting a signal from bound labeled receptor or a signal        from free labeled receptor; and,    -   5) associating the signal from free or bound labeled receptor        with the known amounts of the protein in the composition.

In yet another aspect the present invention relates to a method forcalibrating an assay for a receptor tyrosine kinase and/or a vascularendothelial growth factor in a sample comprising the steps of:

-   -   1) preparing a composition comprising a known or pre-determined        amount of the receptor tyrosine kinase and a known or        pre-determined amount of the vascular endothelial growth factor        either or both altered as described above to reduce or        substantially eliminate their mutual recognition and binding;    -   2) contacting the composition with a receptor specific for a        first epitope of the receptor tyrosine kinase and/or a receptor        specific for a first epitope of the vascular endothelial growth        factor, thereby forming first complexes of receptor specific for        the first epitope of the receptor tyrosine kinase and receptor        tyrosine kinase and/or receptor specific for the first epitope        of the vascular endothelial growth factor and endothelial growth        factor;    -   3) contacting complexes formed in step 2) with a labeled        receptor specific for a second epitope of the receptor tyrosine        kinase and/or a labeled receptor specific for a second epitope        of the endothelial growth factor; thereby forming second        complexes comprising receptor specific for the first epitope of        receptor tyrosine kinase, receptor tyrosine kinase and labeled        receptor specific for the second epitope of the receptor        tyrosine kinase and/or receptor specific for the first epitope        of endothelial growth factor, endothelial growth factor and        labeled receptor specific for the second epitope of endothelial        growth factor;    -   4) separating bound labeled receptor specific for receptor        tyrosine kinase from free labeled receptor specific for receptor        tyrosine kinase and/or bound labeled receptor specific for        endothelial growth factor from free labeled receptor specific        for endothelial growth factor;    -   5) detecting a signal from bound labeled receptor specific for        receptor tyrosine kinase or a signal from free labeled receptor        specific for receptor tyrosine kinase; and/or,    -   6) detecting a signal from bound labeled receptor specific for        endothelial growth factor or a signal from with free labeled        receptor specific for endothelial growth factor; and,    -   7) associating the signal from free or bound labeled receptor        specific for receptor tyrosine kinase and/or the signal from        free or bound labeled receptor specific for endothelial growth        factor with the known amounts of receptor tyrosine kinase and/or        endothelial growth factor in the composition.

In a preferred embodiment the receptor tyrosine kinase is sFlt-1 and theendothelial growth factor is PlGF-1, which PlGF-1 has been altered tohave alanine in place of:

-   -   a) proline at position 25 of SEQ ID NO:1, or    -   b) glutamine at position 27 of SEQ ID NO:1, or    -   c) cysteine at position 60 of SEQ ID NO:1, or    -   d) aspartate at position 72 of SEQ ID NO:1 or    -   e) glutamate at position 73 of SEQ ID NO:1, or    -   f) asparagine at position 84 of SEQ ID NO:1, or    -   g) proline at position 98 of SEQ ID NO:1, or    -   h) tyrosine at position 100 of SEQ ID NO:1, or        altered PlGF-1 having glycine in place of cysteine at position        70 of SEQ ID NO:1, or any combination of the alanine        replacements in a) to h) and the glycine replacement.

In a more preferred embodiment the receptor tyrosine kinase is sFlt-1and the endothelial growth factor is PlGF-1, which PlGF-1 has beenaltered to have alanine in place of aspartate at position 72 of SEQ IDNO:1 and alanine in place of glutamate at position 73 of SEQ ID NO:1.

In another more preferred embodiment, the receptor tyrosine kinase issFlt-1 and the endothelial growth factor is PlGF-1, which PlGF-1 hasbeen altered to have glycine in place of cysteine at position 70 of SEQID NO:1, alanine in place of aspartate at position 72 of SEQ ID NO:1,and alanine in place of glutamate at position 73 of SEQ ID NO:1.

In yet another aspect the present invention relates to a method fordetermining the amount or confirming the presence of a protein in asample comprising the steps of:

-   -   a) contacting the sample with immobilized receptor specific for        a first epitope of the protein, thereby forming a first complex        comprising immobilized receptor specific for the first epitope        of the protein and the protein;    -   b) contacting the first complex with labeled receptor specific        for a second epitope of the protein, thereby forming a second        complex of receptor specific for the first epitope of the        protein, the protein, and labeled receptor specific for the        second epitope of the protein;    -   c) separating labeled receptor that is bound in the second        complex from free labeled receptor;    -   d) determining a signal from labeled receptor that is bound in        the second complex or a signal from free labeled receptor;    -   e) contacting immobilized receptor specific for the first        epitope of the protein with a composition as described above        comprising a known or pre-determined amount of the protein,        thereby forming a third complex comprising immobilized receptor        specific for the first epitope of the protein and the protein of        the composition;    -   f) contacting the third complex with labeled receptor specific        for the second epitope of the protein, thereby forming a fourth        complex of receptor specific for the first epitope of the        protein, the protein of the composition, and labeled receptor        specific for the second epitope of the protein;    -   g) separating labeled receptor that is bound in the fourth        complex from free labeled receptor;    -   h) determining a signal from labeled receptor that is bound in        the fourth complex or a signal from free labeled receptor; and,    -   i) comparing the signals determined in d) and h) as confirmation        of the presence of the protein or as a measure of the amount of        the protein in the sample.

In a preferred embodiment the protein to be determined is a vascularendothelial growth factor. In a more preferred embodiment the protein tobe determined is PlGF and particularly PlGF-1 and the second protein inthe composition is a receptor tyrosine kinase. In a more preferredembodiment the protein to be determined is PlGF-1 and the second proteinin the composition is sFlt-1. In a more preferred embodiment the proteinto be determined is PlGF-1, the second protein in the composition issFLt-1 and PlGF-1 of the composition has been altered to comprisealanine in place of:

-   -   a) proline at position 25 of SEQ ID NO:1, or    -   b) glutamine at position 27 of SEQ ID NO:1, or    -   c) cysteine at position 60 of SEQ ID NO:1, or    -   d) aspartate at position 72 of SEQ ID NO:1 or    -   e) glutamate at position 73 of SEQ ID NO:1, or    -   f) asparagine at position 84 of SEQ ID NO:1, or    -   g) proline at position 98 of SEQ ID NO:1, or    -   h) tyrosine at position 100 of SEQ ID NO:1, or        altered PlGF-1 having glycine in place of cysteine at position        70 of SEQ ID NO:1, or any combination of the alanine        replacements in a) to h) and the glycine replacement.

In a more preferred embodiment the protein to be determined is PlGF-1,the second protein of the composition is sFlt-1 and PlGF-1 of thecomposition comprises alanine in place of aspartate at position 72 ofSEQ ID NO:1 and alanine in place of glutamate at position 73 of SEQ IDNO:1.

In another more preferred embodiment, the protein to be determined isPlGF-1, the second protein of the composition is sFlt-1 and PlGF-1 ofthe composition comprises glycine in place of cysteine at position 70 ofSEQ ID NO:1, alanine in place of aspartate at position 72 of SEQ IDNO:1, and alanine in place of glutamate at position 73 of SEQ ID NO:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates ELISA-based binding of PlGF-1 variants to thesoluble portion of Flt-1. Binding of PlGF-1 variants to human Flt-1,coated at 0.5 μg/ml on a 96-well plate, was performed using increasingconcentrations of soluble proteins ranging between 1 and 16 ng/ml. Wildtype PlGF-1 was used as a positive control.

FIG. 1B illustrates ELISA-based binding of PlGF-1 variants to thesoluble portion of Flt-1. Percentage of binding of PlGF-1 variants at aconcentration of 8 ng/ml calculated with respect to the binding of wtPlGF-1. The results shown represent the average of three independentexperiments.

FIG. 2 illustrates the purity of three PlGF recombinant proteins. Silverstain (A: lanes 1, 2 and 3) and Western blot by monoclonal Rat-4 (B:lanes 4, 5 and 6). Lane assignment: M:SeaBlue ladder (Invitrogen), lane1,4—P126(DE), lane 2,5—P126 (AA) and lane 3,6—P126(GAA).

DETAILED DESCRIPTION

Although the present invention will be described in terms of certainpreferred embodiments relating to pre-eclampsia and biomarker proteinssFlt-1 and PlGF-1, it should be understood that the invention relates toany protein composition and its use in which one or more componentproteins of the composition have been altered to reduce their mutualrecognition and binding.

Whether the pre-eclampsia biomarker proteins are determined using asingle assay platform or a single kit, or determined separately inindependent assays or kits, it is advantageous to have a control orcalibrator comprising both biomarker proteins together in the sameformulation having known or pre-determined concentrations and desiredconcentration ratios. There are at least two problems associated withusing sFlt-1 and PlGF together in native or unaltered form to preparereference, calibrator or control compositions: firstly, sFlt-1 and PlGFbind to each other through a specific binding domain present on eachprotein, as already noted, and secondly, in the serum of mid- tolate-term pregnant women, sFlt-1 is typically present at a significantexcess relative to PlGF whether or not they are afflicted withpre-eclampsia. Unmodified or native PlGF combined and stored togetherwith unmodified or native sFlt-1 will not serve satisfactorily in acomposition used to calibrate an assay for detection of PlGF or sFlt-1because of the nearly quantitative binding of PlGF to sFlt-1.

Amino acid changes have been made to PlGF that reduce or substantiallyeliminate mutual recognition and binding of sFlt-1 and PlGF (Errico etal J. Biol. Chem. 279, 43929-43939, 2004). These amino acid changes donot have a significant impact on the overall protein structure of PlGF.Binding epitopes remain intact and permit these altered proteins to becombined and stored together with sFlt-1 in a composition for use as areference, calibrator or control for assays designed to detect unalteredor native PlGF, unaltered or native sFlt-1, or both.

Targeting amino acid modifications, deletions or replacements to PlGF inorder to reduce or substantially eliminate binding to sFlt-1 has beenfacilitated because the amino acid sequence of PlGF and 3-D crystalstructure are available.

In general it would be advantageous to know secondary, tertiary, andquaternary structures, post-translational modifications (egphosphorylation, glycosylation, sulfation, and ubiquitination), 3-Dcrystal structures of binding proteins and 3-D crystal structures of theproteins engaged in their association complex. However, this informationis not required in order to practice the present invention. Althoughmodification, deletion or replacement of groups associated withpost-translational modifications can be carried out, modification,deletion or replacement of one or more amino acids of one or more of theproteins that engage in mutual recognition and binding is preferred.Site-specific chemical modification of proteins is well known in the art(Techniques in Protein Modification, Lundblad R L, CRC Press, 1995;Chemical Reagents for Protein Modification, Lundblad, R L, CRC Press,3^(rd) Ed, 2005). Chemical/synthetic modification of amino acids can beused to practice the present invention. A preferred approach involvesgenetic engineering techniques. Obtaining the amino acid sequence of aprotein directly is standard practice in the art. Similarly, it isstandard practice in the art to obtain the amino acid sequence of aprotein indirectly from the nucleotide sequence of the gene that codesfor the protein. The nucleotide sequence of a gene can be readilyobtained. And, when the gene is available site-directed mutagenesis canbe carried out to delete, replace, or modify one or more amino acids.This can be done in a random manner or in a predetermined manner. Aprotein that is altered or mutated using site-directed mutagenesis canbe cloned and made readily available. Protein and genetic engineeringdetails and protocols are readily available from numerous publicationsand citations therein (Molecular Cloning, Sambrook J and Russell D W,Cold Spring Harbor Laboratory Press, 2002; Recombinant Gene ExpressionProtocols, Tuan R S ed, Humana Press, 1997; Methods in Molecular Biologyand Protein Chemistry, Spangler B D, John Wiley & Sons Ltd. 2002;Genetic Engineering Fundamentals, Kammermeyer K and Clark V L, MarcelDekker Inc, 1989; Mayo et al, Nature v 306, p 86-88, 1983; Suggs et al,Proc Nat Acad Sci USA v 78, p 6613-6617 1981; Scott et al Nature v 302,p 538-540, 1983; Helfman et al, Proc Nat Acad Sci USA, v 80, p 31-35,1983; Young et al, Proc Nat Acad Sci USA, v 80, p 1194-1198, 1983; U.S.Pat. No. 4,237,224; U.S. Pat. No. 4,273,875; U.S. Pat. No. 4,293,652;U.S. Pat. No. 4,870,009).

The altered protein can be tested to determine if mutual recognition andbinding with its partner protein(s) have been reduced or substantiallyeliminated. This can be carried out using experimental protocols wellknown in the art. The altered protein also can be tested to determine ifepitopes have been sufficiently undisturbed compared with unaltered ornative protein using epitope specific receptors/antibodies. Affinity canbe characterized quantitatively or qualitatively. (Errico et al, J BiolChem, v 279, p 43929-43939, 2004; Piehler et al, J Immunol Methods, v201(2), p 189-206, 1997; Casasnovas et al, v 270, p 13216-13224, 1995;Boone et al, J Virol, v 11, p 515-519, 1973; U.S. Pat. No. 7,081,346;U.S. Pat. No. 5,324,633; U.S. Pat. No. 4,340,668; US 2005/0175999).

Whatever the nature of the group or groups (amino acids and/or non-aminoacids) altered, or the nature of the protein alteration—modification(direct chemical modification—oxidation, reduction, etc), deletion orreplacement of the group(s), or whether one or each of the proteins thatparticipate in mutual recognition and binding are altered, the twoimportant functional features are: 1) mutual recognition and binding ofan altered protein to an unaltered partner protein or binding of partnerproteins when each have been altered is such that mutual recognition andbinding is sufficiently reduced or substantially eliminated and 2) oneor more epitopes of any altered protein retain binding propertiessufficiently similar or substantially identical to the epitope(s) in theunaltered or native protein if this property is required for theparticular application as discussed earlier.

Example I

Human PlGF-1 and variants, sFlt-1, anti-human PlGF-1 antibodies directedto human PlGF, binding characteristics of PlGF and variants to sFlt-1,ELISA assay for determining PlGF and other materials and experimentalprotocols have been described by Errico et al, J Biol Chem, v 279, p43929-43939, 2004, and are reproduced herein in part. The Errico et alreference can be consulted for details regarding cell cultures,plasmids, selection of cell lines, and other materials and experimentalprotocols not explicitly provided herein.

Materials

As described by Errico et al., anti-human PlGF monoclonal antibodies andhuman Flt-1 (Flt-1/Fc chimera) are available from R&D Systems(Minneapolis, Minn. USA). Goat anti-mouse IgG-horseradish peroxidase(HRP) is available from Santa Cruz Biotechnology (Santa Cruz, Calif.USA; www.scbt.com).

Construction of PlGF Variants

Errico et al. obtained PlGF variants using PCR techniques carried outusing the plasmid named pchPlGF-1 as template and PCR was performedusing complementary primers mapping the region encoding the amino acidto be mutated to alanine and bearing the specific nucleotidemodification. For the preparation of the PlGF variant having the doublemutation, primers carrying both mutations were utilized. Amplified DNAwas purified and used to transform competent bacteria. The plasmids weresequenced in both directions using the dideoxynucleotide method. Thefollowing PlGF-1 single residues were mutated to Ala: Asn-16, Pro-25,Gln-27, Cys-60, Asp-72, Glu-73, Asn-74, Asn-84, Pro-98, and Tyr-100. Thedouble mutant Asp 72 to Ala and Glu 73 to Ala of PlGF-1 was alsogenerated.

Calibrator/Control Compositions Comprising Altered PlGF-1

Calibrators/controls comprising altered PlGF-1 and sFlt-1 are preparedby combining unaltered sFlt-1 with an altered PlGF-1, in particular, thedouble mutant in which alanines replace aspartate at position 72 of SEQID NO:1 and glutamate at position 73 of SEQ ID NO:1 or the triple mutantin which there is an additional mutation of glycine replacing cysteineat position 70. These may be combined individually from dry formpreparations or from working aqueous stock solutions prepared using anysuitable buffer at a desired pH (such as, phosphate in saline (PBS), pH7.5) comprising any other addenda that may be useful or required—such asanti-oxidants, preservatives, etc. For illustrative purposes, theconcentration of altered PlGF-1 is in the range of 0 to about 1000pg/mL, and sFlt-1 fixed at 100 pg/mL but other concentration ranges forboth may be used. The unaltered sFlt-1 is combined with altered PlGF ofthe double mutant or the triple mutant in PBS (10 g NaCl, 0.25 g KCl,1.8 g Na₂HPO₄, 0.3 g KH₂PO₄, pH 7.5) to produce the following set ofreference, calibrator or control materials:

Altered PlGF1 (pg/mL) SFlt-1 (pg/mL) 0 100 50 100 100 100 500 100 1000100

ELISA For PlGF

The quantity of PlGF in a sample of serum obtained from a pregnant womanis determined using an ELISA for PlGF. The ELISA (described in detailbelow) is calibrated using the set of solutions comprising alteredPlGF-1 and sFlt-1 described above. The signal observed for each PlGF-1level of the set is associated with the concentration of altered PlGF-1.The association can be represented in graphic form or correlated usingappropriate statistical and mathematical calibration methods. The signalobserved in the ELISA assay using the serum sample is compared with thecalibration graph to determine the concentration of PlGF in the sampleor transformed into concentration units using the establishedmathematical association.

The ELISA is carried out as follows: for determination of PlGF in asample, one anti-human PlGF-1 monoclonal antibody at 1 μg/ml in PBS isused to coat a 96-well plate at 100 μl/well and incubated overnight at4° C. The wells are washed once with PBS containing 0.05% TWEEN 20 (PBT)and non-specific binding sites are blocked by introducing 1% bovineserum albumin in PBS at 280 μl/well and incubation for 3 h at roomtemperature (RT). The wells are aspirated and kept in the cold untiluse. During the assay, 100 μl of each calibrator level or serum sampleis appropriately diluted in PBET (PBS containing 0.1% bovine serumalbumin, 5 mM EDTA, 0.05% Tween 20) and incubated for 1 hour at 37° C.The wells are washed five times by PBT and another anti-human PlGF-1monoclonal antibody (this one HRP conjugated) diluted in PBET at 37ng/ml, is added to the wells and incubated for 1 h at 37° C. The wellsare washed five times with PBT and 100 μl of HRP substrate composed of 1mg/ml of orthophenylenediamine in 50 mM citrate phosphate buffer, pH 5and 0.006% H₂O₂ is added and incubated for 30 min in the dark at RT. Thereaction is stopped by adding 25 μl/well of 4 N H₂SO₄, and the signalabsorbance is measured at 490 nm on a microplate reader.

Comparison of Altered PlGF-1 and Unaltered PlGF Binding to sFlt-1

Errico et al. has described the experiment to determine the binding ofaltered PlGF-1 and unaltered/native PlGF-1 to Flt-1. Basically, a96-well plate is coated with a soluble human Flt-1 (Flt-1/Fc chimera) at0.5 μg/ml in PBS, pH 7.5, 100 μl/well, overnight at RT. The plate iswashed five times with PBT, and after the blocking non-specific sites ofwells with bovine serum albumin solution as described above, the bindingreaction is allowed to proceed by adding altered PlGF-1 orunaltered/native PlGF to a well and incubating for 1 h at 37° C. and 1 hat RT. The wells are washed with PBT as described above and incubatedwith a biotinylated anti-human PlGF-1 polyclonal antibody, 300 ng/ml inPBET, for 1 h at 37° C. and 1 h at RT. Detection is performed asdescribed above in the ELISA assay and the signals obtained with alteredPlGF-1 and unaltered/native PlGF-1 are compared. The results obtained byErrico et al. are reproduced in FIG. 1A and FIG. 1B.

Example II Comparing Recombinant PlGF (DE) and PlGF (AA) ExperimentPurpose:

Two recombinant PlGF proteins were evaluated (1) for their bindingreactivity to monoclonal antibody specific to human PlGF, and (2) fortheir binding reactivity to sFlt, the formation of ligand:receptorcomplex.

Materials and Reagents: (1) Recombinant PlGF:

Two versions of purified recombinant PlGF were used. The proteinconsists of a 21-amino-acid leader sequence that does not belong toPlGF. The leader sequence contains a “6×His” tag and a 4-amino-acid Xarecognition and cleavage site.

SEQ ID NO: 2: Leader sequence: MRGS HHHHHH GSGSGSG IEGRThe PlGF portion sequence in PlGF (DE): Amino acid sequence correspondsto wild-type PlGF amino acids 4-132 of SEQ ID NO:1, resulting in thefollowing DE amino acid sequence:

SEQ ID NO: 3: AVPPQQWALS AGNGSSEVEV VPFQEVWGRS YCRALERLVDVVSEYPSEVE HMFSPSCVSL LRCTGCCG DE  NLHCVPVETANVTMQLLKIR SGDRPSYVEL TFSQHVRCEC RPLREKMKPE RCGDAVPRRThe PlGF portion sequence in PlGF (AA): Amino acid sequence correspondsto wild-type PlGF amino acids 4-132 of SEQ ID NO:1 with two mutationsmade at amino acid positions 72 and 73 in SEQ ID NO:1, resulting in thefollowing AA amino acid sequence:

SEQ ID NO:4 : AVPPQQWALS AGNGSSEVEV VPFQEVWGRS YCRALERLVDVVSEYPSEVE HMFSPSCVSL LRCTGCCG AA  NLHCVPVETANVTMQLLKIR SGDRPSYVEL TFSQHVRCEC RPLREKMKPE RCGDAVPRR(2) Recombinant sFlt:

Full length sFlt was obtained from Scios Inc. (Mountain View, Calif.USA; www.sciosinc.com) (Lot #9225-89), consists of 687 amino acids ofsoluble fms-like tyrosine kinase 1 (sFlt-1).

sFlt-1 sequence:

SEQ ID NO: 5:MVSYWDTGVLLCALLSCLLLTGSSSGSKLKDPELSLKGTQHIMQAGQTLHLQCRGEAAHKWSLPEMVSKESERLSITKSACGRNGKQFCSTLTLNTAQANHTGFYSCKYLAVPTSKKKETESAIYIFISDTGRPFVEMYSEIPEIIHMTEGRELVIPCRVTSPNITVTLKKFPLDTLIPDGKRIIWDSRKGFIISNATYKEIGLLTCEATVNGHLYKTNYLTHRQTNTIIDVQISTPRPVKLLRGHTLVLNCTATTPLNTRVQMTWSYPDEKNKRASVRRRIDQSNSHANIFYSVLTIDKMQNKDKGLYTCRVRSGPSFKSVNTSVHIYDKAFITVKHRKQQVLETVAGKRSYRLSMKVKAFPSPEVVWLKDGLPATEKSARYLTRGYSLIIKDVTEEDAGNYTILLSIKQSNVFKNLTATLIVNVKPQIYEKAVSSFPDPALYPLGSRQILTCTAYGIPQPTIKWFWHPCNHNHSEARCDFCSNNEESFILDADSNMGNRIESITQRMAIIEGKNKMASTLVVADSRISGIYICIASNKVGTVGRNISFYITDVPNGFHVNLEKMPTEGEDLKLSCTVNKFLYRDVTWILLRTVNNRTMHYSISKQKMAITKEHSITLNLTIMNVSLQDSGTYACRARNVYTGEEILQKKEITIRGEHCNKKAVFSRISKFKSTRNDCTTQ SNVKH(3) Monoclonal Antibody to Human sFlt-1 and to Human PlGF:

monoclonal Ab ID Source Cat # Lot Clone RD-1 mouse anti-sFlt RD SysCGG07605B 49560 RD-2 mouse anti-sFlt RD Sys BYC01605A 49543 Rat-1 ratanti-PlGF RD Sys n/a 1103925 358903 Rat-2 rat anti-PlGF RD Sys n/a1103933 358939 Rat-3 rat anti-PlGF RD Sys n/a 1103931 358932 Rat-4 ratanti-PlGF RD Sys n/a 1103926 358905 Rat-5 rat anti-PlGF RD Sys n/a1103927 358907 MS-1 mouse anti-PlGF RD Sys MAB264 n/a 37203

Experiment Examples

(1) ELISA assay-1:

-   -   High-binding microtiter plate was coated with recombinant        PlGF(DE) or PlGF(AA) at 0.5 ug/mL and blocked with BSA/PBS    -   Standard ELISA procedure consists of monoclonal antibody        dilution in casein/PBS; dilution of HRP conjugated donkey        anti-mouse IgG or donkey anti-rat IgG at 1:3 K in casein/PBS;        100 uL/well sample or conjugate volume; each step incubation at        37 C/30 min/shake; 6 times plate washing, 100 uL OPD substrate        development for 25 C/30 min; 25 uL stop solution; record OD at        492 nm.    -   ELISA results assay 1 are shown in Table 1.

TABLE 1 Recognition of monoclonal anti-PlGF to recombinant PlGF(unaltered and altered) Antibody binding activity to coated PlGF (OD)Monoclonal anti-PlGF ID, clone # and Coated concentration (ng/mL)recombinant Ms-1 Rat 1 Rat 2 Rat 4 Rat 3 Rat 5 PlGF Ab (10 ng/mL) Ab(100 ng/mL) (0.5 μg/mL) 37203 358903 358939 358905 358932 358907 PlGF-1(DE) 1.823 2.098 2.237 2.114 1.233 1.256 PlGF-1 (AA) 2.650 1.789 2.2331.935 1.305 1.297

-   -   Conclusion: All monoclonal antibodies tested reacted to both        PlGF(DE) and PlGF(AA), indicating that D72/E73A mutation did not        affect monoclonal antibody binding and these antibody epitope        locations were not at these two mutation sites.

(2) ELISA Assay-2:

-   -   High-binding microtiter plate was coated with recombinant        PlGF(DE) or PlGF(AA) at 0.5 ug/mL and blocked with BSA/PBS    -   Standard ELISA procedure consists of 1st plate incubation with        diluted sFlt in casein/PBS at various concentrations; 2nd plate        incubation with mixed anti-sFlt solution comprising two        monoclonal antibodies of RD-1 and RD-2 each at 0.1 μg/mL; 3rd        plate incubation with HRP conjugated donkey anti-mouse at IgG at        1:4 K dilution in casein/PBS; and 4th plate incubation with 100        μL OPD substrate development for 30 min at 25° C. 1st, 2nd and        3rd plate incubation steps are for 15-20 min/shake at 37° C.; 6        times plate washing between each step. 25 μL stop solution after        4th incubation and record OD at 492 nm.    -   ELISA results assay-2 are shown in Table 2.

TABLE 2 Binding of sFlt to recombinant PlGF (unaltered and altered)Complex formation of Coated sFlt to coated PlGF recombinant sFltconcentration PlGF (ng/mL) at incubation (0.5 μg/mL) 1800 600 200 66.67BSA (control) 0.004 0.006 0.010 0.020 PlGF-1 (DE) 1.728 1.204 0.5240.316 PlGF-1 (AA) 0.150 0.080 0.060 0.040 * Bound sFlt:PlGF complex weredetected by mouse anti-sFlt and HRP anti-mouse conjugate

-   -   Conclusion: sFlt formed receptor:ligand complex with coated        PlGF(DE). However, such complex formation was greatly reduced        with PlGF(AA) mutant, indicating that amino acid positions 72        and 73 in SEQ ID NO:1 were critical for sFlt-1 binding and        complex formation.

(3) Biacore Assay:

-   -   sFlt were immobilized on Biacore chip FC-2 via NHS/EDC coupling        to a RU=6738. FC-1 was blank as negative control.    -   PlGF(DE) or PlGF(AA) were injected to FC-1 and FC-2 to evaluate        complex formation    -   Biacore results are shown in Table 3.

TABLE 3 Biacore measurement of sFlt:PlGF complex recomb hu PlGF 20 μg/mLCM5 Chip: PlGF (DE) PlGF (AA) Bicore FC1: blank 11 12 (RU) FC2: sFlt 15630 FC2 − FC1 145 18

-   -   Conclusion: Injected PlGF(DE) bound to immobilized sFlt to form        receptor:ligand complex while injected PlGF(AA) bound to        immobilized sFlt poorly.

Example III

Comparison between Wild-type Recombinant PlGF and Two Additional PlGFMutants

Experiment Purpose:

Three additional recombinant PlGF proteins were constructed andevaluated (1) for their binding reactivity to monoclonal antibodyspecific to human PlGF, and (2) for their binding reactivity to sFlt,the formation of ligand:receptor complex.

Materials and Reagents:

(1) Three Additional Recombinant PlGF Proteins were Constructed asFollows:

P126(DE): recombinant P1GF wild type: SEQ ID NO: 6:MRGSAVPPQQWALSAGNGSSEVEVVPFQEVWGRSYCRALERLVDVVSEYPSEVEHMFSPSCVSLLRCTGCCGDENLHCVPVETANVTMQLLKIRSGDRPSYVELTFSQHVRCECRPLREKMKPERCGDAVGP GQIVGGVYLLThe first four amino acid residues are unrelated amino acids (MRGS); thelast ten amino acids are the 10 G epitope (C terminal tag); the twoamino acids preceding the 10 G epitope are also unrelated amino acids(Gly-Pro); the 126 amino acid sequence between the unrelated amino acids(i.e. beginning after MRGS and preceding GP) is the PlGF sequenceidentical to amino acid positions 4 to 129 in SEQ ID NO:1.

P126(AA): P1GF mutant#1: SEQ ID NO: 7:MRGSAVPPQQWALSAGNGSSEVEVVPFQEVWGRSYCRALERLVDVVSEYPSEVEHMFSPSCVSLLRCTGCCG AA NLHCVPVETANVTMQLLKIRSGDRPSYVELTFSQHVRCECRPLREKMKPERCGDAVGP GQIVGGVYLLsame as P126(DE) except the underlined amino acids (AA) are the twomutated amino acids

P126(GAA): P1GF mutant#2: SEQ ID NO: 8:MRGSAVPPQQWALSAGNGSSEVEVVPFQEVWGRSYCRALERLVDVVSEYP SEVEHMFSPSCVSLLRCTGCG G AA NLHCVPVETANVTMQLLKIRSGDRPS YVELTFSQHVRCECRPLREKMKPERCGDAVGPGQIVGGVYLLsame as PlGF mutant #1 P126(AA) except an additional mutation at theamino acid two before AA is mutated from C to G

All three recombinant proteins were expressed in bacteria and all forminsoluble inclusion bodies. After sonication, washing with 4 M urea inPBS and 2M urea in PBS, inclusion bodies were finally solubilized by 8Murea/15 mM reduced Glutathione (GSH)/50 mM Tris-HCL (pH7.8). PlGFproteins were refolded through three step dialysis: (1) 24 hours againstdialysis buffer 3M urea/50 mM TRIS(pH7.5)/2 mM EDTA/0.2 M Arginine/2 mMGSH, (2) 24 hours against dialysis buffer 2M urea/50 mM TRIS(pH7.5)/2 mMEDTA/0.2 M Arginine/1.2 mM GSH/0.4 mM oxidized Glutathione (GSSG) and(3) 24 hours against dialysis buffer 0.8M urea/20 mM TRIS(pH7.5)/2 mMEDTA/0.2 M Arginine/0.48 mM GSH/0.16 mM GSSG. Refolded PlGF were furtherpurified by loading dialyzed protein solution to an affinity column,prepared by cross linking monoclonal antibody specific to 10 G tag andCNBr-activated Sepharose 4 Fast Flow resin (GE catalogue #17-0981-01).The bound PlGF was then eluted by 40% acetonitrile. Purified PlGFproteins were finally obtained after buffer exchange to PBS.

(2) Monoclonal anti-human PlGF, monoclonal anti-human sFlt are availablefrom R&D Systems (Minneapolis, Minn. USA); HRP conjugated donkeyanti-rat IgG (Cat #712-035-150) are available from JacksonImmunoResearch Laboratories, Inc. (West Grove, Pa., USA); ELISA plateswere Costar hind binding by Corning Life Sciences (Cat #2592);Electrophoresis gels NuPAGE 4-12%, PVDF transfer membrane and SeeBlueladder were from Invitrogen (Carlsbad Calif., USA); Blocker casein/PBSand SuperSignal West Dura western blot substrate were purchased fromPierce (Rockford, Ill., USA). CNBr-activated Sepharose 4 Fast Flow resin(Cat #17-0981-01) and Silver stain kit (Cat #17-1150-01) were from GEHealthcare (Piscataway, N.J., USA)

Experiment Examples

(1) Recombinant PlGF: FIG. 2 shows the purity of the three PlGFrecombinant proteins by silver stain and western blot.

(2) ELISA Assay-1

-   -   High-binding microtiter plate was coated with recombinant        P126(DE) or P126(AA) or P126(GAA) at 0.5 ug/mL and blocked with        BSA/PBS    -   Standard ELISA procedure consists of monoclonal antibody        dilution in casein/PBS; dilution of HRP conjugated donkey        anti-mouse IgG or donkey anti-rat IgG at 1:3 K in casein/PBS;        100 uL/well sample or conjugate volume; each step incubation at        37 C/30 min/shake; 6 times plate washing, 100 uL OPD substrate        development for 25 C/30 min; 25 uL stop solution; record OD at        492 nm.    -   ELISA results are shown in Table 4.

TABLE 4 Recognition of monoclnal anti-PlGF to recombinant PlGF(unaltered and altered) Antibody binding activity to coated PlGF (OD)Monoclonal anti-PlGF ID, clone # and Coated concentration (ng/mL)recombinant Ms-1 Rat 1 Rat 2 Rat 4 Rat 3 Rat 5 PlGF Ab at 1 ng/mL Ab at10 ng/mL (0.5 ug/mL) 37203 358903 358939 358905 358932 358907 P126 (DE)1.023 1.368 1.497 1.634 0.779 0.834 P126 (AA) 0.967 1.301 1.633 1.6810.681 0.697 P126 (GAA) 1.134 1.226 1.530 1.591 0.806 0.799

-   -   Conclusion: All monoclonal antibodies tested reacted to        P126(DE), P126(AA) and P126(GAA), indicating that D72A/E73A        double mutation and C70G/D72A/E73A triple mutation did not        affect monoclonal antibody binding and these antibody epitope        locations were not at these mutation sites.

(3) ELISA Assay-2:

-   -   High-binding microtiter plate was coated with recombinant        P126(DE), P126(AA) and P126(GAA) at 0.5 ug/mL and blocked with        BSA/PBS    -   Standard ELISA procedure consists of 1^(st) plate incubation        with diluted sFlt in casein/PBS at various concentration, 2^(nd)        plate incubation with mixed anti-sFlt solution comprising two        monoclonal antibodies of RD-1 and RD-2 each at 0.1 ug/mL, 3rd        plate incubation with HRP conjugated donkey anti-mouse IgG at        1:4 K dilution in casein/PBS and 4^(th) plate incubation with        100 uL OPD substrate development for 25 C/30 min. 1^(st), 2^(nd)        and 3^(rd) plate incubation step is at 37 C/15-20 min/shake; 6        times plate washing between each step. 25 uL stop solution after        4^(th) incubation and record OD at 492 nm.    -   ELISA results are shown in Table 5.

TABLE 5 Binding of sFlt to recombinant PlGF (unaltered and altered)Complex formation of sFlt to coated PlGF sFlt concentration Coatedrecombinant (ng/mL) at incubation PlGF at (5 ug/mL) 1800 600 200 66.67BSA (control) 0.009 0.008 0.005 0.003 P126 (DE) >3 1.833 0.833 0.347P126 (AA) 0.210 0.182 0.115 0.143 P126 (GAA) 0.150 0.101 0.095 0.088 *Bound sFlt:PlGF complex were detected by mouse anti-sFlt and HRPanti-mouse conjugate

-   -   Conclusion: sFlt formed receptor:ligand complex with coated        P126(DE). However, such complex formation was greatly reduced        with P126(AA) mutant and P126(GAA) mutant, indicating that amino        acid position 70, 72 and 73 in SEQ ID NO:1 were critical for        sFlt-1 binding and complex formation.

(4) ELISA Assay-3:

-   -   High-binding microtiter plate was coated with recombinant sFlt        at 0.5 ug/mL and blocked with BSA/PBS    -   Standard ELISA procedure consists of 1^(st) plate incubation        with diluted P126(DE) or P126(AA) or P126(GAA) in casein/PBS at        various concentration, 2^(nd) plate incubation with monoclonal        anti-PlGF Rat-4 solution at 0.1 ug/mL, 3^(rd) plate incubation        with HRP conjugated donkey anti-Rat IgG at 1:4 K dilution in        casein/PBS and 4^(th) plate incubation with 100 uL OPD substrate        development for 25 C/30 min. 1^(st), 2^(nd) and 3^(rd) plate        incubation step is at 37 C/15-20 min/shake; 6 times plate        washing between each step. 25 uL stop solution after 4^(th)        incubation and record OD at 492 nm.    -   ELISA results are shown in Table 6.

TABLE 6 Binding of recombinant PlGF (unaltered and Complex formation ofPlGF (unaltered or altered) to Coated PlGF (ng/mL) at sFlt at (0.5 1000500 250 100 0 P126 (DE >3 1.388 0.557 0.259 0.021 P126 (A 0.299 0.1180.101 0.077 0.009 P126 (GA 0.119 0.117 0.069 0.088 0.051 * BoundsFlt:PlGF complex were detected by mouse anti-PlGF (Rat-4) and conjugat

-   -   Conclusion: unaltered PlGF, P126(DE), formed ligand:receptor        complex with coated sFlt. However, altered PlGF (P126(AA) and        P126(GAA) failed to form such complex, indicating that amino        acid position 70, 72 and 73 in SEQ ID No:1 were critical for        sFlt binding and complex formation.

The description of the specific embodiments of the invention ispresented for the purposes of illustration. It is not intended to beexhaustive or to limit the scope of the invention to the specific formsdescribed herein. It will be understood by one of ordinary skill in theart that various modifications can be made without departing from thespirit and scope of the invention as set forth in the claims.

All patents, patent applications, and publications cited herein arehereby incorporated by reference.

1. A method of calibrating or running controls in an assay for a firstprotein and a second protein, the method comprising: selecting as acalibrator or control a composition comprising known amounts of thefirst protein and the second protein, wherein native first and nativesecond proteins form a non-covalent association complex, and wherein oneor more amino acids or one or more non-amino acid groups of the firstprotein, or the second protein, or the first protein and the secondprotein, have been deleted, modified, or replaced with a different aminoacid or non-amino acid group or groups thereby reducing or substantiallyeliminating formation of a non-covalent association complex between thefirst protein and the second protein; and calibrating or runningcontrols for the assay using the selected composition.
 2. The method ofclaim 1 wherein the calibrating or running controls comprises:contacting the composition with a first receptor specific for the firstprotein and a second receptor specific for the second protein, therebyforming a first complex of the first protein and the first receptor, anda second complex of the second protein and the second receptor; anddetecting amounts of the first complex and the second complex, whereinamounts of the first complex and the second complex are compared to theknown amounts of the first protein and the second protein to calibrateor control the assay.
 3. The method of claim 2 wherein one of the firstprotein and the first receptor is labeled with a first detectable label,and one of the second protein and the second receptor is labeled with asecond detectable label, and the detecting comprises detecting the firstdetectable label and the second detectable label.
 4. The method of claim2 wherein the detecting comprises contacting the first complex with afirst labeled moiety and the second complex with a second labeledmoiety, thereby forming a labeled first complex and a labeled secondcomplex, and wherein the detecting comprises detecting the labeled firstcomplex and the labeled second complex.
 5. The method of claim 1 whereinthe first protein is a receptor tyrosine kinase and the second proteinis a vascular endothelial growth factor.
 6. The method of claim 5wherein the receptor tyrosine kinase is sFlt-1 and the vascularendothelial growth factor is PlGF.
 7. The method of claim 6 wherein thePlGF is PlGF-1.
 8. The method of claim 7 wherein PlGF-1 comprisesalanine in place of: a) proline at position 25 of SEQ ID NO:1, or b)glutamine at position 27 of SEQ ID NO:1, or c) cysteine at position 60of SEQ ID NO:1, or d) aspartate at position 72 of SEQ ID NO:1 or e)glutamate at position 73 of SEQ ID NO:1, or f) asparagine at position 84of SEQ ID NO:1, or g) proline at position 98 of SEQ ID NO:1, or h)tyrosine at position 100 of SEQ ID NO:1, or altered PlGF-1 havingglycine in place of cysteine at position 70 of SEQ ID NO:1, or anycombination of the alanine replacements in a) to h) and the glycinereplacement.
 9. The method of claim 7 wherein PlGF-1 comprises alaninein place of aspartate at position 72 of SEQ ID NO:1 and alanine in placeof glutamate at position 73 of SEQ ID NO:1.
 10. The method of claim 7wherein PlGF-1 comprises glycine in place of cysteine at position 70 ofSEQ ID NO:1, alanine in place of aspartate at position 72 of SEQ ID NO:1and alanine in place of glutamate at position 73 of SEQ ID NO:1.
 11. Amethod of calibrating an assay for a protein in a sample comprising thesteps of: 1) contacting a composition with an immobilized receptorspecific for the protein, wherein the composition comprises knownamounts of the protein and the second protein, wherein native proteinand native second protein form a non-covalent association complex, andwherein one or more amino acids or one or more non-amino acid groups ofthe protein, or the second protein, or the protein and the secondprotein, have been deleted, modified, or replaced with a different aminoacid or non-amino acid group or groups thereby reducing or substantiallyeliminating formation of a non-covalent association complex between theprotein and the second protein, thereby forming a complex comprisingimmobilized receptor and the protein of the composition; 2) contactingthe complex formed in step 1) with a labeled receptor specific for theprotein, thereby forming a complex comprising immobilized receptor,protein of the composition and labeled receptor; 3) separating boundlabeled receptor from free labeled receptor; 4) detecting a signal frombound labeled receptor or a signal from free labeled receptor; and, 5)associating the signal from free labeled receptor or bound labeledreceptor with the known amount of the protein in the composition. 12.The method of claim 11 wherein one of the protein and the second proteinis a vascular endothelial growth factor and the other is a receptortyrosine kinase.
 13. The method of claim 12 wherein the vascularendothelial growth factor is PlGF and the receptor tyrosine kinase issFlt-1.
 14. The method of claim 13 wherein the PlGF is PlGF-1.
 15. Themethod of claim 14 wherein the PlGF-1 comprises alanine in place of: a)proline at position 25 of SEQ ID NO:1, or b) glutamine at position 27 ofSEQ ID NO:1, or c) cysteine at position 60 of SEQ ID NO:1, or d)aspartate at position 72 of SEQ ID NO:1 or e) glutamate at position 73of SEQ ID NO:1, or f) asparagine at position 84 of SEQ ID NO:1, or g)proline at position 98 of SEQ ID NO:1, or h) tyrosine at position 100 ofSEQ ID NO:1, or altered PlGF-1 having glycine in place of cysteine atposition 70 of SEQ ID NO:1, or any combination of the alaninereplacements in a) to h) and the glycine replacement.
 16. The method ofclaim 14 wherein PlGF-1 comprises alanine in place of aspartate atposition 72 of SEQ ID NO:1 and alanine in place of glutamate at position73 of SEQ ID NO:1.
 17. The method of claim 14 wherein PlGF-1 comprisesglycine in place of cysteine at position 70 of SEQ ID NO:1, alanine inplace of aspartate at position 72 of SEQ ID NO:1 and alanine in place ofglutamate at position 73 of SEQ ID NO:1.